The technical field of the invention is the manufacturing of three-dimensionally shapable galvanic structures on substrates, having their possibilities and limitations according to prior art by structuring with a photosensitive resist and subsequent galvanic deposition.
Prior art describes the forming of galvanic structures, which are electrically conductive and find an extensive field of application in micromechanics, by coating with a photosensitive resist, by exposing and removing of certain parts of the applied resist and subsequent galvanic deposition from a galvanic fluid which is switched to a first potential, whereas the electrically conductive basis on the substrate as plating base is switched to an opposite potential. Galvanic structures obtained by said method generally have edge portions extending in vertical direction, as the incident light on the photosensitive resist always is perpendicular, so that only perpendicular resist structures can develop. Thus, only such galvanic structures may be provided, the flanks of which extend in the direction the inciding light had.
The object of the invention is to give up resist structures predetermined by the inciding light as a pattern for galvanic structures and to provide the possibility of obtaining freely shapable galvanic structures the side surfaces (flanks) of which no longer being forced to extend in vertical or substantially vertical direction. The solutions proposed by the present invention are specified according to the invention disclosed.
Accordingly, structures which are shapable in vertical direction and galvanically deposited are provided as profiles on a substrate by structuring a conductive layer, which is applied before, in lateral direction. Said three-dimensionally shapable metallic structure provided by a galvanization method is also obtained if the substrate is provided with at least one conductive layer, one of said number of or the one conductive layer is structured in lateral direction by forming a number of island areas having no or no substantial conduction with respect to each other, and during galvanization, one of said island areas is provided as a core area and switched to a cathodic potential. The result obtained by said manufacturing method steps disclosed herein.
On the basis of the product obtained by said method, which product is a three-dimensionally shaped solid metallic structure with a more than distinct extension in vertical direction (xe2x80x9cprofilexe2x80x9d), it may be recognized that no longer all vertical side surfaces of said structure are forced to extend in vertical or substantially vertical direction. However, this does not mean that all sides are compelled to have a beveled or curved extension, but only that at least one of the vertical flanks is a beveled or curved surface which could not be shaped by using a known manufacturing method. The xe2x80x9cprofilexe2x80x9d according to the present invention is not a surface in the form of a laterally deposited layer.
Certainly, a photosensitive resist is still used, however, it is no longer the core and the basis for a galvanic deposition on the basis of exposed and etched guiding structures based on a photosensitive resist layer. According to the invention, said resist is used for shaping a three-dimensional structure in vertical direction by covering a major part of the core area and leaving only a small section free, thus permitting a cathodic potential to pass over into the galvanic bath. Starting from said potential in the galvanic fluid, a slowly growing layer is formed on said small section with cathodic potential. Said layer grows in vertical direction thereby also being forced to slightly enlarge in lateral direction.
As the invention provides further island areas, which are spaced in lateral direction, as second areas which are obtained from the plating base after their structuring, which plating base was at first deposited as a continuous conductive layer, an over-galvanization over isolated areas from the cathodic area to the subsequent structured area which is spaced apart therefrom and again electrically conductive but not switched to the cathodic potential. Said manner of controlling the over-galvanization in lateral direction permits to define the beveled surface dependent on the interspace and the width of said electrically not connected island areas, said beveled surface being obtained by a longer or shorter galvanization time.
Each respective front of said beveled surface growing in lateral directionxe2x80x94which is the growing profilexe2x80x94always grows discontinuously (stepwise) when reaching as an electrically conductive layer the subsequent conducting island. The subsequent expansion in lateral direction is much faster than when just reaching the end of said island area in lateral direction. Controlled by the stepwise lateral expansion and the substantial growth of the conducting structure thereby obtained, a solid structure of any shape may be provided on the substrate which structures so far could not be obtained by coating with resist, uncovering and usual galvanization methods.
In short words, the new method permits to control the lateral growth rate by determining the length and duration of the galvanization process on the one hand andxe2x80x94before starting the galvanization processxe2x80x94by applying defined geometric shapes as structures to the plating base subjected to the galvanic bath during galvanization.
The galvanic profile results from the ratio of the lateral growth rate with respect to the vertical growth rate.
The galvanic structures may be selected to comprise one of gold and nickel. The electrically conductive starting layer (plating base) may comprise the same materials. If a number of plating bases is used, they are isolated by intermediate layers, preferably SiN layers. If only one plating base is used, the substrate itself may have isolating function, not requiring an additional isolating layer to be provided on the substrate.
If the manufacturing method according to the invention does not use thick resist layers, no lateral frame is formed for those vertical surfaces which are not intended to be beveled. In this case, it is possible to let the beveled galvanic structure grow in all directions, e. g. in circular or ellipsoidal form. If concentric island areas are used for controlling the lateral growth speed, also concave or convex profiles may be obtained having a substantially unlimited height h.
If a thick resist structure is used, a free growth rate in lateral direction, defined by a stepped over-galvanization may only develop in those areas where the structured photosensitive resist does not define a limit. In the areas being provided with a thick resist layer, the growth rate is limited, the galvanic deposition being provided only in a direction where no limitation by resist is defined.
The coating with thick resist structures may advantageously be combined with a completely free plating base structure, so that in certain areas, the growth rate in lateral direction (x- and y-direction) is limited and localized, however, in another direction it is predetermined due to the structured plating base.
The inventive method is very inexpensive, particularly with respect to a known gray-tone lithographic method, this technology being based on reducing the size of the structures in the mask design in relation to the lithographic resolution. Thus, gray-tone lithography does no longer provide bright/dark areas but a mixture of gray tones which, however, may only be achieved by using a very complex mask design requiring an optical reduction device for exposure (compare xe2x80x9cMicrofabrication of complex surface topographiesxe2x80x9d in Sensors and Actuators, A 46-47, pages 89-94, 1995). In contrast to a gray-tone lithographic method which at first provides a three-dimensional resist profile, subsequently being coated with a thin metallic layer to permit a galvanic deposition, the method according to the present invention provides solid galvanic structures without resist layers positioned thereunder and without using exposure stepper devices, but only applying simple and wide-spread UV contact exposure devices. Said devices are considerably less expensive and thus the results obtained with them are also less expensive.
If, according to the present invention, x-ray lithographic methods (LIGA technique) are used for structuring the thick resist, three-dimensional structures of a height of several 100 xcexcm may be obtained. If a UV photosensitive resist and contact exposure devices are used, the height of the thick (structured) resist layer is limited to about 100 xcexcm due to the maximum exposure depth of said method. The limit thus provided only applies for manufacturing methods using thick photosensitive resists, those inventive manufacturing methods which work without resist limits as edge areas of the galvanic are not restricted in their height by said exposure depth, the height depending solely on the period of time during which galvanic deposition is effected.